Method of sweetening petroleum products



Patented May 12, 1942 E METHOD OF SWEETENING PETROLEUM PRODUCTS Donald R. Stevens, Swissvale, and Joseph E.

Nickels, Pittsburgh, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Application March 2, 1939, Serial No. 259,486

2 Claims.

This invention relates to the production of a doctor sweetened petroleum distillate having improved resistance to oxidational changes; and it is more particularly concerned with a method of producing a doctor sweetened gasoline having improved response to antiknock and antioxidant compounds and improved color stability comprising treating gasoline containing mercaptans with a solution of sodium plumbite in amount sufficient to convert the reactive mercaptans to lead mercaptides, and with sulfur in an amount sufficient to convert all of said mercaptides to disulfides and insufficient to break the lead sulfidegasoline suspension thus formed, and thereafter treating said suspension with a solution of asettling agent having a concentration suificient to increase the settling rate of said suspension without rendering said gasoline sour; all as more fully hereinafter set forth and as claimed.

The process of doctor sweetening is applied to sour gasoline-and other petroleum products which are corrosive and bad smelling because of the presence in them of substantial amounts of mercaptans. Nearly all of these mercaptans will react readily and are converted by the doctor. treatment to disulfides which are less objectionable in odor and are non-corrosive. The doctor sweetening treatment consists in mixing the sour gasoline with an alkaline plumbite solution which reacts with mercaptans to produce lead mercaptides, and adding sulfur which reacts with the mercaptides to produce alkyl disulfides. The order of treatment with these reagents is not material and the sulfur may be added to the gasoline either before or after the plumbite treatment. The alkaline plumbite solution customarily used is a solution of sodium plumbite, and while various alkali and alkaline earth plumbite solutions may be used, our invention will be described hereinafter more particularly with reference to the use of a sodium plumbite solution;

A suitable sodium plumbite solution may be made, for example, by saturating a per cent solution of caustic soda with litharge. The sodium. plumbite reacts with the reactive mercaptans on treatment of the sour gasoline to form lead mercaptides and caustic soda according to the equation:

The sulfur added to the gasoline reacts with the mercaptides to form alkyl disulfides and lead sulfide according to the reaction:

than the untreated gasoline.

While, as shown by this reaction, a single atom of sulfur is sufiicient to convert a molecule of lead mercaptide to disulfide and form insoluble lead sulfide, it has been found in practice that when sulfur is added in this proportion the lead sulfide formed does not precipitate but tends to be colloidally suspended in the gasoline and to settle out very slowly. It is customary therefore in commercial operation of this process to add sufficient sulfur to cause the so-called break or precipitation of the lead sulfide and clarification of the gasoline, which amount is substantially in excess of that required to convert the lead mercaptides to alkyl disulfides. great an excess of sulfur will make the gasoline corrosive and it is customary to keep the excess of sulfur added between the amount which will produce the break and the amount which will impart to the gasoline the property of darkening a copper strip under test conditions.

Cracked gasolines which have been doctor sweetened in this way exhibit a loss in oxygen stability value and a lessened response to oxidation inhibitors together with a decrease in color stability. Furthermore, both straight run and cracked gasolines'doctor sweetened in this manner have a lessened response to antiknock compounds, particularly tetra-ethyl lead. I

The decrease in oxygen stability and inhibitor response of gasoline resulting from a doctor sweetening treatment in which sulfur is added in an amount sufficient tobreak the lead sulfidegasoline suspension is apparently caused by alkyl polysulfides formed in the gasoline by reaction of the excess sulfur with the mercaptides previously formed. We have found that maximum oxygen stability and inhibitor response is obtained when just enough sulfur has been added to convert all of the lead mercaptides present in the gasoline to alkyl disulfides and lead sulfide. With the addition of this amount of sulfur the gasolinebecomes sweet and has a higher oxygen stability As additional sulfur is added the oxygen stability and the inhibitor response of the sweet gasoline gradually decrease until at the point at which sulfur breaks the lead sulfide-gasoline suspension the resulting gasoline has a lowered oxygen stability and inhibitor response which may be less than those of the original unsweetened gasoline. I

A desirable object achieved by the present invention is to provide a method of doctor sweetening gasoline in which less than enough sulfur is added to'produce the break and a clarified sweetened gasoline is obtained having improved The addition of too solution of sodium plumbite, in an amount sufiicient to convert all of the reactive mercaptans present in said gasoline to lead mercaptides and with sufiicient sulfur to convert all of the lead mercaptides thus formed to alkyl disulfides with the formation of a semi-colloidal suspension of lead sulfide in the gasoline. We then treat the gasoline containing the lead sulfide in suspension with a solution of an agent adapted to cause the lead sulfide to migrate out of the gasoline phase and into the aqueous phase at an'increased rate,

without souring the gasoline. We have found that aqueous solutions of a number of inorganic and organic compounds are effective to produce a substantially complete separation of the lead sulfide from suspension in the hydrocarbon phase in a relatively short time and when these solutions are used in proper concentration the gasoline separated from the lead sulfide remains sweet to the doctor test. These agents are referred to herein and in the appended claims as settling agents.

Among the compounds which we have found most satisfactory as settling agents are sodium carbonate, sodium tetra-borate, sodium chloride, potassium nitrate, ferrous sulfate, barium sulfide and calcium sulfide. Aqueous solutions of these compounds containing them in amounts up to the saturation point may be used to break the lead sulfide-gasoline suspension without turning the gasoline sour again.

In addition, we have found that aqueous solutions of sodium sulfide, mono-sodium phosphate, di-sodium phosphate, sodium bicarbonate, sodium hydroxide, potassium ethyl xanthate, ferric sulfate, calcium chloride, thiourea, cane sugar and gelatin are effective as settling agents to increase the rate of settling of a lead sulfide-gasoline suspension. However, solutions of these latter compounds are not desirable in all concentrations. For example, di-sodium phosphate solutions containing 1.0 percent or more of the compound tend to emulsify with the gasoline and it is generally advantageous to use this compound in concentrations less than 1.0 per cent. Sodium hydroxide solutions at ornear the saturation point cause less rapid separation of the suspension than less concentrated solutions, and it is generally not advantageous to use concentrations greater than about 10 per cent. Sugar solutions also have a tendency to give less rapid separation when used in very high concentrations, although 7 concentrations up to 10 per cent have given good results. The remainder of these compounds,

namely sodium sulfide, mono-sodium phosphate, sodium bicarbonate, potassium ethyl xanthate, ferric sulfate, calcium chloride, thiourea and gelatin have a tendency to render the gasoline sour again if used in too concentrated solution.

It is necessary when using these latter compounds that the concentration of the solution used be maintained below a maximum value, which is specific for each system, in order to prevent the gasoline from becoming sour again. For example, sodium sulfide may be used in solutions having concentrations less than 0.5 per cent; mono-sodium phosphate solutions having concentrations less than 0.2 per cent will not sour ing concentrations up to 10 per cent.

the gasoline; sodium bicarbonate is advantageously used in concentrations less than 5.0 per cent; potassium ethyl xanthate solutions will not sour the gasoline in concentrations up to 1.0 per cent; ferric sulfate solutions having concentrations up to 5 per cent may be used; and calcium chloride is advantageously used in solutions hav- Of the organic compounds we have found suitable for the purposes of our invention gelatin may be used in concentrations up to 1 percent and thiourea in concentrations up to 5 per cent.

In carrying out the method of our invention it is first necessary to determine the amount of sulfur which will be required to convert all of the mercaptides, formed from the reactive mercaptans in the gasoline by the sodium plumbite solution, to alkyl disulfides and lead sulfide without adding excess sulfur to the gasoline. This value may be determined in various ways. 'One method is to titrate a mixture of gasoline and sodium plumbite solution with a standard solution of sulfur in gasoline, benzol or the like until the sample is sweet to the doctor test. A gasoline is considered sweet to the doctor test when no discoloration appears on shaking it with one half of its volume of doctor solution for 15 seconds, adding sufiicient free sulfur to cover the inter-face of the two solutions and shaking again for 15 seconds. The discoloration is due to the formation of lead sulfide and the doctor test is quite sensitive. This method of titrating with a standard solution of sulfur in gasoline is, however, somewhat difficult, since a doctor test must be made at each step ofthe titration.

We have generally found it more advantageous, therefore, to determine the reactive mercaptan content of the gasoline. The mercaptan content of gasoline is not completely reactive and. is therefore not completely responsive to the doctor treatment. The difference between the reactive mercaptan content and the total mercaptan content is however quite small and is the result of the fact that some of the higher molecular weight mercaptans are too strongly soluble in the hydrocarbon phase to react readily. While the total mercaptan content may be determined by titration with silver nitrate, the amount thus determined is somewhat larger than the amount that reacts in the doctor treatment. It is important for the purposes of this invention that the mercaptans of the gasoline which will react in the doctor treatment, which are referred to herein and in the appended claims as reactive mercaptans, be determined. One method we have found satisfactory for determining the reactive mercaptans is to titrate a sample of the gasoline to be treated, with a standard cupric ammonium sulfate solution. The proper amount of sulfur to be added to convert all of the reactive mercaptans to disulfides may be calculated according to the equations recited above.

After the proper amount of sulfur to be added has been determined the gasoline to be sweetened may be treated in the usual manner with the sodium plumbite solution and with the calculated amount of sulfur. This process is ordinarily practiced continuously. In such case the gasoline and sodium plumbite solution are pumped together through a mixing device and a stock solution of sulfur in some of the same gasoline is pumped into the mixture in such proportion as .to add the calculated amount of sulfur. Various modifications of this procedure may be employed. The gasoline may be titrated at intervals in this continuous operation or a stock tank maybe titrated and the introduction of the stock sulfur solution regulated to provide the proper amount of sulfur.

After separation from the doctor solution, the sweet gasoline thus obtained, containing suspended lead sulfide, is contacted according to'the method of our invention with a solution of a settling agent. For example, the suspension may be agitated with a suitablequan'tity of the solution of the settling agent :either in a batch operation or continuously and the .mixture then allowed to separate into an aqueous and a hydrocarbon layer. of these two layers that the suspended lead sulfide has migrated from the :gasoline and settled out in the aqueous layer. Often complete separation can be efiected by agitation for one minute, although longer periods, for example about ten minutes, are generally more advantageous. The solution of the settling agent may be used in amounts as low as per cent by volume of the gasoline-lead sulfide suspension and particularly good results have been obtained using an amount corresponding to about 50 per cent by volume of the gasoline-lead sulfide suspension.

The gasoline obtained after settling out the lead sulfide in this manner is color stable and sweet to the doctor test, has a high-oxygen sta- It'will be found upon separation bility factor and is readily responsive to antialkyl disulfides during the doctor sweetening treatment, a sweet gasoline having a better response to antioxidant and fantiknock compounds than gasoline sweetened by a process in which sufficient sulfur is added to break the lead sulfide-gasoline, will be obtained if an amount of sulfur somewhat greater than that required to convert all of the lead mercaptides to alkyl disulfides but less than the amount of sulfur which will break the lead sulfide-gasoline suspension is used. Our invention contemplates therefore the use of amounts of sulfur between the amount required to convert all of the lead mercaptides to alkyl disulfides and the amount required to break the lead sulfide-gasoline suspension so formed, as well as amounts of sulfur just enough to convert all of the lead mercaptides to alkyl disulfides.

Substantially any of the known antioxidant compounds may be added to a gasoline which,

has been sweetened according to the method of our invention with improved results. We have obtained particularly good results with the alkali insoluble alkylated phenol antioxidants such as 2,6-di-tertiary-butyl-4-methyl-phenol, 2,4,6-tritertiary butyl phenol, 2,4,6-tri-tertiary-amylphenol and the like. The stabilized gasoline thus obtained is resistant to oxidational changes over a substantially longer period of time than a cracked gasoline doctor sweetened in the manner heretofore known and practiced in the art. It is possible also to produce a stabilized gasoline using less antioxidant compound when the gasoline has been doctor sweetened according to the method of our invention.

Similarly, a gasoline doctor sweetened according to the method of our invention exhibits improved response to antiknock compounds. By the addition of tetra-ethyl lead to such a gasoline, a product having a better Iantiknock rating is obtained than "when a like amount of tetraethyl lead is added to the same raw .gasoline sweetened by a doctor sweetening method in which enough sulfur is added to break the lead sulfide-gasolinesuspension.

-The color stability or a sweet gasoline produced according to the method of "our invention is also better than the color stability of gasoline sweetened according to methods in which excess sulfur is used. That is to say, there is less tendency for such gasoline to become discolored upon exposure to sunlight than sweetened gasoline in the production of which an excess of sulfur was used. This 'efiect 'is shown in the following table:- TABLE I Effect 'of sweetening upon color stability, sulfur required- (titmted) '=0.'002

v Saybolt color Treatment Doctor test At Two Five Tel-1 days start days days g i None Sour. 22 l9 l8 1 Dtoctor+0.002% sul- S 7 ur weet 22 18 D ?ctor+0.004%sul- +16 +16 or Sweet. +22 16 9 7 Doctor+0.006% sulfur Sweet. +21 +15 +7 **1 *Correct amount. Orange-brown sediment appeared on bottom of sample bottle.

The above samples were not inhibited.

According to the Saybolt color scale, higher numbers indicate less color, and decreasing number's, progressive coloration.

In the following comparative example there-is shown the improvement in oxygen stability and in response to antioxidant and antiknock compounds of a gasoline doctor sweetened according to the method of our invention, over the same gasoline given a doctor sweetening treatment in which enough sulfur is added to break the lead sulfide-gasoline suspension. An unsweetened clay treated high temperaturecracked gasoline which had been washed with a 5.0 per cent solution of sodium carbonate to remove any hydrogen suli de present was titrated to determine its reactive mercaptan content. Th titration was carried out bycarefully adding a standard copper ammonium sulfate solution from a burette to 100 grams of the raw sour distillate. The standard copper ammonium sulfate solution was prepared as described by Krause in The Chemist Analyst (Baker 8: Adamson) 2'7, No. 1, page 14, (1938), and contained 1 mg. of cupric copper per cubic centimeter so that each cubic centimeter used in titration represented 0.001 per cent of mercaptan sulfur present in the gasoline. Exactly 5.0 cc. of the copper ammonium sulfate solution were required to produce the blue coloration which indicates the end-point, indicating that the gasoline sample contained 0.005 per cent sulfur as reactive mercaptans. This reactive mercaptan content calls for the addition of 0.00% per cent of elementary sulfur to just convert all of the mercaptans to disulfides in the sweetening process.

treatment with a borax solution to settle out the lead sulfide from suspension in the gasoline. The treatment consisted of agitating 500 volumes of sour distillate, to which 0.0.025 per cent of sulfur had been added with one-halt its volume 01' a sodium plumbite solution formed by saturating a 15.0 per cent caustic soda, solution with litharge, for ten minutes. At the end of this period the doctor solution was separated from the black lead sulfide-gasoline suspension. The suspension was then agitated with one-half its volume of a saturated sOlutiOn of sodium tetraborate for ten minutes. The gasoline layer upon separation from the sodium tetraborate solution was perfectly clear and sweet, while the borax solution was darkened by the lead sulfide.

Two other samples of the same gasoline were then sweetened in processes in which 0.0075 per cent and 0.0150 per cent sulfur were added.

The octane number of each sample with and without the addition of 2 cubic centimeters per gallon of tetra ethyl lead was then determined. Likewise, the oxygen stability period of each sample with the addition of 0.04 gram of 2,4,6-tritertiary-butyl-phenol per 100 cubic centimeters as an antioxidant, was measured.

- The following table gives the results obtained:

it is equally applicable to the sweetening of any petroleum distillates or other hydrocarbon products containing reactive mercaptans and w do not intend that our invention shall be limited except as hereinafter defined in the appended claims.

What We claim is:

1. A method of producing a doctor sweetened gasoline having improved resistance to oxidational changes and improved response to antioxidant and antiknock compounds comprising treating gasoline containing mercaptans with a solution of sodium plumbite in an amount sufficient to convert the reactive mercaptans to lead mercaptides, and with sulfur in an amount just suflicient to convert said'mercaptides to alkyl disulfides, and thereafter treating the lead sulfidegasoline suspension thus formed with a solution of sodium tetraborate having a concentration sufilcient to increase the settling rate of said suspension without rendering said gasoline sour.

2. A method of producing a doctor sweetened gasoline having improved resistance to oxida- Correct amount of sulfur.

By the term settling agent as used herein and in the appended claims we mean to include those compounds which in aqueous solution act to cause lead sulfide suspended in gasoline to migrate from the gasoline into said solution, without reacting with the lead sulfide itself so that the lead sulfide may be removed from suspension in gasoline without leaving any deleterious compounds in the gasoline or causing the gasoline to become sour.

While we have particularly described this invention with respect to the doctor sweetening treatment of gasoline it will be understood that tional changes and improved response to antioxidant and antiknock compounds comprising treating gasoline containing mercaptans with a solution of sodium plumbite in an amount sufiicient to convert the reactive mercaptans to lead mercaptides and with sulfur in an amount just suflicient to convert said mercaptides to alkyl disulfides and thereafter treating the lead sulfidegasoline suspension thus formed with a saturated solution of sodium tetraborate.

DONALD R. STEVENS.

JOSEPH E. NICKELS. 

